An example for the simple training of M3GNet formation energy model is added

[kenko911]

Summary

An example for the simple training of M3GNet formation energy model is added

Checklist

• [x ] Google format doc strings added. Check with ruff.
• [ x] Type annotations included. Check with mypy.
• [ x] Tests added for new features/fixes.
• [ x] If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)

Tip: Install pre-commit hooks to auto-check types and linting before every commit:

pip install -U pre-commit
pre-commit install

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (9aa74a0) 97.97% compared to head (07e5b04) 97.97%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #176   +/-   ##
=======================================
  Coverage   97.97%   97.97%           
=======================================
  Files          28       28           
  Lines        1828     1828           
=======================================
  Hits         1791     1791           
  Misses         37       37           

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